Gas turbine powered aircraft operate in many areas of the world and consequently encounter many different environmental conditions. In desert area flight operations, large quantities of airborne sand particles significantly affect engine performance. Such sand enters a gas turbine engine primarily during takeoff and landing, accumulating within the engine by adhering to the blades, vanes, and other internal engine components. In the high temperature engine sections, where temperatures may exceed 1000.degree. C., a layer is gradually deposited on the various components as the entering sand effectively bonds to the hot component surfaces. The presence of these deposits decreases overall engine efficiency by increasing engine weight, modifying airfoil surface shapes, roughening smooth aerodynamic surfaces, and, with some types of dust, corrosively damaging critical engine components. Such a decrease in engine efficiency results in reduced engine thrust at a given engine speed. Typically, an engine must operate at higher speeds to compensate for the reduced thrust, thereby increasing fuel consumption and engine maintenance requirements.
In the hot turbine section of an engine, coatings are generally used to enhance the oxidation and hot corrosion resistance of superalloy articles. An aluminide coating, such as that disclosed in commonly assigned U.S. Pat. No. 4,132,816 to Benden et al, is exemplary of a typical protective coating. While the exact nature of the chemical bond is unknown, desert sand, such as that encountered in Dhahran, Saudi Arabia, is adhesive to such protective coatings, building up over a period of time on the hot coated surfaces and eventually flaking off due to thermal stress on engine cool down. Generally, a portion of the protective coating flakes off with the deposit. The cyclic build-up and flaking of these deposits on a coated surface eventually removes the protective coating, leading to failure of the substrate superalloy article.
Frequent removal of desert sand deposits from internal engine components is required to prevent such engine damage and to maintain optimum engine efficiency. Commercial detergent solutions are available for cleaning dirt deposits from the internal component surfaces of a gas turbine engine. However, these solutions are generally formulated for removing oil and dirt deposits from the cold compressor section of a gas turbine engine and have proven ineffective in removing sand deposits from the surfaces of such superalloy articles as the airfoil blades and vane clusters located in the hot engine sections.
Another method for removing dirt accumulations from the internal component surfaces of a gas turbine engine involves the introduction of abrasive particles in the airflow path of the engine. Such particles are carried through the engine by the flowing airstream, generally eroding any deposits on the engine surfaces by striking the deposits at high velocity. U.S. Pat. No. 4,065,322 to Langford discloses such a procedure in which carbon based particles are introduced into the airflow path of an engine while running. This procedure has several limitations. First, it is difficult to assure even distribution of the abrasive particles within the engine during operation. The flow of air through an engine, particularly a bypass type turbine engine, is highly complex, producing eddys and currents as the air flows around engine components. Since the flowing air carries the cleaning particles into these eddys and currents, uniform particle distribution and velocity cannot be maintained. Consequently, several areas of the engine are not cleaned while other areas are overly attacked by the flowing particles. Another limitation involves the accumulation of loosened debris and abrasive particles within the engine, thereby exchanging one deposit for another. This is a particular problem with airfoil blades having air cooling passages. The langford disclosure discusses particles which essentially vaporize at hot engine temperatures thereby first cleaning the engine and then vaporizing any trapped carbon particles left behind after cleaning. However, since both the cleaning particles and loosened debris from the compressor section are traveling through the aft turbine section, a mixture of material may enter and block the cooling passages, thereby reducing cooling regardless of the vaporization ability of the cleaning particles.
The most certain way to assure proper engine cleaning is to frequently overhaul engines used in desert environments. Of course, such a procedure requires removal of the engine from the aircraft, dissembling the engine into its component parts, cleaning such parts by grit blasting or soaking in special solutions and then reassembling the engine. Such a procedure is quite costly and time consuming, requiring excessive aircraft downtime.